MICHIGAN FORESTS FOREVER TEACHERS GUIDE

WINTER ADAPTATIONS OF ANIMALS

Winter is the most stressful time of year in the north for most forms of life.  The key hardships are a lack of food and cold temperatures.   However, don't let a reduction in activity appear as if there is nothing going on in the woods!

A lack of food occurs for at least two reasons, both related to low temperatures. The first reason has to do with a reduction in active plant life.  Plants, of course, are the sources of nearly all food chains.  The second reason has to do with availability.  For many animals, food sources are buried under snow or ice.  Deep snow is not a problem for all creatures.  To field mice, it is a protective layer against most predators.  To predators, deep snow means a time of going hungry. 

Specialized adaptation to winter involves exploring chemistry, physics, and animal behavior.  Managing an energy budget is the key to survival.  There are many ways to manage this budget, primarily through combinations of physical attributes (morphology, habitat, and behavior) and physiological capabilities (body chemistry and metabolic controls). 

How Do Animals Respond to Cold Winters?

There are three main strategies to surviving inclement conditions, migration, dormancy, and toughing it out.  Each species is suited to a particular variant of one strategy or the other, or a combination of strategic elements. 

1.  Migration and Movement.  Many species migrate between seasons.  Some, such as the arctic tern, travel 10,000 miles between winter and summer habitats.  It's difficult to ignore the migration of geese, cranes, and ducks . . . and difficult to believe that monarch butterflies actually migrate to Mexico.  How in the world do tiny hummingbirds fly all the way across the Gulf of Mexico?  The return of the colorful and vociferous warblers becomes obvious in the Spring, but their departure in the Fall is generally missed.  The first Spring bluebird is noted by many . . . but few can mark their departure date. 

Migration is not always a dramatic, long-distance affair.   Other species, such as white-tailed deer, move to areas that are more survivable.  Deer pretty much vacate the Lake Superior watershed during the deep snow season.  Biologists have been able to track some of these migration patterns in the U.P.  Reptiles and amphibians move to protected places underground or under water to avoid freezing temperatures.  Fishes will move to different waters.  Most of us have noted the indoor migration of the Asian ladybird beetle! 

For those people who prefer to be indoors most of the winter, the outdoors may appear to be uniformly cold and uncomfortable.  However, there are many microclimates where winter stress is significantly lower.  Logs, caves, holes, dead trees, spruce and cedar stands, under snow, and human structures are examples of places that provide shelter from winter extremes.  These are critical places for wildlife.

Not all migrators leave Michigan, either.  Some actually migrate TO Michigan for the winter or on a cyclical basis!  Chickadees and great gray owls are two good examples.  The playful, curious, and nearly fearless whisky-jack makes its presence well-known at camps and many winter feeders.  During lows in the snowshoe hare population cycles, Canada lynx may roam into the U.P. in search of food.  We need to remember that our winters are not as severe as we sometimes boast about.  There is a large land mass to our north where winters are considerably longer and colder!

2.  Dormancy.   There are several forms of dormancy as the taxonomic groups are surveyed.  Definitions are challenging due to the many variations of dormancy.  There has been a lot of research into how animals cope with inclement weather, winter in this case.   

Torpidity is a controlled reduction of body metabolism, evidenced by low oxygen consumption rates and lower body temperatures.  A key part of the definition is accurate metabolic control.   It is a phenomenon restricted to warm-blooded animals.  Cold-blooded animals experience different physiology in response to adverse conditions.  Some animals will undergo daily states of torpidity as a response to a lack of food and in combination with other environmental conditions.  Other species undergo seasonal torpidity.  In the north, hibernation is the most dramatic form.  Torpidity is not restricted to northern species and can be found in the tropics, too.  Estivation is a kind of torpidity in very hot and dry conditions. 

Many northern species undergo metabolic changes that allow them to "sleep" through the winter.  Sleep, of course, is not what they do, but torpor can superficially appear that way.  The most advanced form of torpor is hibernation.  Hibernation is quite complex and fascinating.  Although definitions are evasive, hibernation is a controlled significant drop in metabolism to a selected level, although the term hibernation is sometimes used for cold-blooded animals and any form of winter dormancy.  Chipmunks, certain mice, ground squirrels, and groundhogs are examples of true hibernators.  Their body temperatures are maintained a few degrees above their ambient environment, which is usually in a place protected from weather extremes.  Hibernators are usually small animals because small animals have high rates of metabolism to begin with.  Increases in these already high rates of metabolism in order to maintain body temperature comes at a metabolic cost that is just too high for some species.

True hibernators cannot be easily "woken up".  They are largely unresponsive to external stimuli.  Generally they maintain only a sufficient amount of specialized fat reserves to carry them through the winter season and to then arouse them during the late winter or early spring.  Arousal is a very expensive metabolic process that they can usually afford to do only a few times, sometimes only once.  Bears do not hibernate, although this continues to be argued.  Their body temperatures drop only a few degrees and metabolism is reduced to only moderate rates.  Female bears give birth during the winter, something that would not be possible for a true hibernator.  Lastly, bears can easily be aroused in the winter and then drop back into a state of torpidity.  Don't be fooled by a "hibernating" bear in its den!

Dormancy in cold-blooded animals is a reduced state of metabolic activity largely controlled by environmental conditions.  Cold-blooded animals must become dormant during the winter because they lack the internal control over their metabolism.  Many seek sheltered places and undergo chemical changes to prevent their tissues from freezing.  Others can tolerate certain levels of ice between cells, commonly in tandem with chemical changes.  Spring peepers, chorus frogs, gray tree frogs, and wood frogs tolerate and regulate a frozen state.  Good snow cover is essential to survival, as they overwinter under leaf litter on the forest floor.  These frogs thaw out in the spring, which is why we hear them sing so early in the season on those increasingly warm evenings. 

Insects overwinter as eggs, pupae, or adults.  Dormancy is often coupled with specialized chemical adaptations to help survive the winter season.  Some have the ability to resist freezing, others can tolerate freezing to certain degrees.  There are also insects that can employ either strategy.  Chemicals associated with dormancy are sugars and certain alcohols such as glycerol, sorbitol, mannitol, and ethylene glycol. 

Plants also experience dormancy but cannot relocate to sheltered places, other than reverting to seeds on the ground and roots under the ground.  Tree adaptations are covered on another page. 

3.  Toughing It Out.  Winter remains an active time of the year because many species have adapted to active lifestyles during the winter.  Cold-blooded animals (amphibian, reptiles, and insects) must find sheltered places where they can ride out the winter without freezing and being eaten by predators.  Fish continue to be active (as ice-fishers know!) but often at a reduced rate.  For some species, the winter energy equation is always negative, meaning they cannot consume or conserve enough energy to survive the winter.  While consumption and conservation are critical, these species must rely on fat reserves and their margin for survival is often slim.  This is part of the reason why long and severe winters can take a heavy toll on wildlife populations whose northern range occurs in Michigan.  

There is a wide array of morphological, physiological, and behavioral adaptations for winter survival.  A few examples are provided below, but investigations into the lives of active winter animals will reveal many combinations of survival strategies.

• Bergmann's Rule states that northern species of a particular genus or similar class of birds or mammals tend to be larger in size, although this is not always true.  Larger body size means a higher body mass-to-surface area ratio.  It's easier to retain heat.  Polar bears are larger than tropical bears.  White-tailed deer in Michigan dress out at higher weights than their counterparts in Texas or Florida.  
• Body appendages tend to get smaller in the north, as a heat conservation measure.  Snowshoe hares have smaller ears that cottontail rabbits.  Mammalian legs and snouts are frequently shorter and stouter.
• Specialized fat, called brown fat, is produced during the food-rich seasons and expended during cold seasons.  This is also the kind of fat that most hibernators use for arousal and many migrators use for fuel.
• Various "heat exchange" mechanisms can be found in animal circulatory systems that reduce heat loss to body extremities.
• Certain fish and herptiles produce chemicals within and between cell walls that can lower their freezing temperature a few degrees.  In sheltered environmental niches, these few degrees can mean the difference between life and death.
• Some mammals, such as bats, flying squirrels and small rodents, will occupy collective dens to conserve body heat, even though some species are non-colonial during the warm season.  This is part of the reason that some species of snakes will do the same thing.
• Food preferences change with the season.  Some browsers, such as white-tailed deer, have changes in digestive enzymes to cope with the different food sources.  This is one of the reasons why biologists argue against winter deer feeding.  If not done correctly, a deer can starve to death with a belly full of corn.
• Ruffed grouse "snow roost" during periods of extreme cold.  Snow provides a very effective barrier against severe cold.  They will rest under the snow until the severe weather passes.  Folks who snowshoe or cross-country ski too close to these snow roosts are often caught off-guard when a grouse explodes out of the snow.  Large piles of grouse droppings are spring-time indicators of how severe the winter was. 
• Aquatic mammals, such as otter and mink, grow thick layers of insulating fat and have specialized fur.  Similarly, ducks, geese, and swans have feathers and oil glands that keep water away from the skin.  Some have efficient circulatory heat exchangers between the body and the feet.  It's usually not the cold that causes waterfowl to migrate.  It's more a matter of food shortages. 
• Birds and mammals undergo seasonal changes in feathers and pelage.  Trappers know that winter pelts are the highest quality because they are thicker and have different kinds of hair.
• Muskrats and beaver construct shelters, partly for protection from severe weather.  A number of animals dig burrows, such as groundhogs, foxes, chipmunks, and moles. 
• Many species of birds can adjust their internal body temperature downward to reduce the temperature gradient with environmental temperatures, thus reducing heat loss.  They also tend to shiver a lot to maintain body temperatures. 

To Winter Adaptations of Trees

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	This website was developed and created by Michigan State University Extension for the teachers of the State of Michigan.